Metabolic Pathways and Defects in Fructose Metabolism

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Recently, fructose has become an increasing part of the Western diet, even though its ingestion can be harmful. Long-term ingestion has effects on diabetes and obesity. The most drastic and common genetic disorder of fructose metabolism is hereditary fructose intolerance (HFI). Lack of knowledge about sites of fructose metabolism and about genotype-phenotype relationships still exists for this disease and reflects the incomplete understanding of normal fructose metabolism. Answers to two major questions will provide new information. First, other than liver and kidney, what other tissues play a role in fructose metabolism? Second, can small molecules be found that stabilize the major defective enzyme in HFI, that harboring an A149P substitution (AP-aldolase)? The proposed investigations will, 1) define sites for fructose assimilation and utilization using a combination of bioinformatics and molecular approaches, 2) determine a high-resolution structure of AP-aldolase, and use it to find stabilizing small-molecule ligands by both structure-based ligand design (SBLD) and high-throughput screening of chemical libraries, 3) create animal models for HFI using gene-targeting techniques, and 4) identify HFI mutations in the diverse US population, in particular Hispanic, African-American, and other ethnic groups that have not been well characterized, and correlate these findings to any specific phenotypes in these ethnic groups. The large database of expressed sequence tags (dbEST) will be analyzed for overlapping expression profiles of the GLUT5, GLUT2, ketohexokinase, aldolase, hexokinase, and triose kinase in both mouse and humans to predict alternative sites of fructose metabolism. Verification and characterization of these global predictions will be done by quantitative reverse-transcriptase polymerase chain reaction (Q-PCR), RNA in situ hybridization (RISH), and identification of metabolic intermediates during the oxidation of radioactive fructose. For the second hypothesis, a high-resolution structure of AP-aldolase will be determined by macromolecular X-ray crystallography. Screening large libraries of small molecules (produced by combinatorial chemistry) using a thermal-stability assay of AP-aldolase in conjunction with SBLD will identify small molecules that restore enzyme function. Should the gene-targeted animal model mimic the human HFI pathology, the metabolic profiles and sites of fructose metabolism will be determined. Should the animal model be asymptomatic, differences in the ability to metabolize fructose will be compared to previously determined sites and pathways for fructose metabolism in normal mice and humans. Lastly, blood samples from African-American and Hispanic-American HFI subjects will be used for identification of gene defects by direct DNA sequencing, thus offering a reliable non-invasive diagnostic method to these Americans.